Which of these numbers is prime? ${15,\ 33,\ 44,\ 49,\ 97}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 15 are 1, 3, 5, and 15. The factors of 33 are 1, 3, 11, and 33. The factors of 44 are 1, 2, 4, 11, 22, and 44. The factors of 49 are 1, 7, and 49. The factors of 97 are 1 and 97. Thus, 97 is a prime number.